The "Minimum" Solar Box Cooker is a solar oven that you can build quickly from two cardboard boxes.

The "Minimum" Solar Box Cooker is a simple box cooker that can be built in a few hours for very little money. When we designed this cooker, we named it the "Minimum Solar Box Cooker" because, at the time, it represented the simplest design we could devise. What we didn't communicate with that name was that this is a full-power cooker that works very well, and is in no way minimum as far as its cooking power goes.

Contents

What You Will Need

Two cardboard boxes. We would suggest that you use an inner box that is at least 38 cm (15 in) by 38 cm (15 in), but bigger is better. The outer box should be larger than the small box all around, but it doesn't matter how much bigger, as long as there is 1.5 cm (0.6 in) or more of an airspace between the two boxes. The distance between the two boxes does not have to be equal all the way around. Also, keep in mind that it is very easy to adjust the size of a cardboard box by cutting and gluing it. Note that you can build the Easy Lid Cooker with a single cardboard box.

One sheet of cardboard to make the lid. This piece must be approximately 4 cm (1.6 in) to 8 cm (3.2 in) larger all the way around than the top of the finished cooker (the outer box).

One can of flat-black spray paint (look for the words "non-toxic when dry") or one small jar of black tempera paint. Some people have reported making their own paint out of soot mixed with wheat paste.

One turkey-size Reynolds Oven Cooking Bag®. These are available in almost all supermarkets in the U.S. and they can be mail-ordered from Solar Cookers International. They are rated for 204 °C (399 °F) so they are perfect for solar cooking. They are not UV resistant; thus they will become more brittle and opaque over time and may need to be replaced periodically. A sheet of glass can also be used, but this is more expensive and fragile, and doesn't offer that much better cooking except on windy days. Plexiglas also works. See Glazing for more possibilities.

Building the Base

Fold the top flaps closed on the outer box and set the inner box on top and trace a line around it onto the top of the outer box. Remove the inner box and cut along this line to form a hole in the top of the outer box (Figure 1).

Decide how deep you want your oven to be. It should be about 2.5 cm (1 in) deeper than your largest pot and about 2.5 cm (1 in) shorter than the outer box so that there will be a space between the bottoms of the boxes once the cooker is assembled. Using a knife, slit the corners of the inner box down to that height. Fold each side down forming extended flaps (Figure 2). Folding is smoother if you first draw a firm line from the end of one cut to the other where the folds are to go.

Glue aluminum foil to the inside of both boxes and also to the inside of the remaining top flaps of the outer box. Don't bother being neat on the outer box, since it will never be seen, nor will it experience any wear. The inner box will be visible even after assembly, so if it matters to you, you might want to take more time here. Glue the top flaps closed on the outer box.

Place some wads of crumpled newspaper into the outer box so that when you set the inner box down inside the hole in the outer box, the flaps on the inner box just touch the top of the outer box (Figure 3). Glue these flaps onto the top of the outer box. Trim the excess flap length to be even with the perimeter of the outer box.

Finally, to make the drip pan, cut a piece of cardboard the same size as the bottom of the interior of the oven and apply foil to one side. Paint this foiled side black and allow it to dry. Put this in the oven so that it rests on the bottom of the inner box (black side up), and place your pots on it when cooking. The base is now finished.

Building the Removable Lid

Take the large sheet of cardboard and lay it on top of the base. Trace its outline and then cut and fold down the edges to form a lip of about 7.5 cm (3 in). Fold the corner flaps around and glue to the side lid flaps (Figure 4). Orient the corrugations so that they go from left to right as you face the oven so that later the prop may be inserted into the corrugations (Figure 6). One trick you can use to make the lid fit well is to lay the pencil or pen against the side of the box when marking (Figure 5). Don't glue this lid to the box; you'll need to remove it to move pots in and out of the oven.

To make the reflector flap, draw a line on the lid, forming a rectangle the same size as the oven opening. Cut around three sides and fold the resulting flap up forming the reflector (Figure 6). Foil this flap on the inside.

To make a prop, bend a 30 cm (12 in) piece of hanger wire as indicated in Figure 6. This can then be inserted into the corrugations as shown.

Next, turn the lid upside down and glue the oven bag (or other glazing material) in place. We have had great success using the turkey size oven bag (47.5 cm x 58.5 cm, 19 in x 23 1/2 in) applied as is, i.e., without opening it up. This makes a double layer of plastic. The two layers tend to separate from each other to form an airspace as the oven cooks. When using this method, it is important to also glue the bag closed on its open end. This stops water vapor from entering the bag and condensing. Alternately you can cut any size oven bag open to form a flat sheet large enough to cover the oven opening.

Improving Efficiency

The oven you have built should cook fine during most of the solar season. If you would like to improve the efficiency to be able to cook on more marginal days, you can modify your oven in any or all of the following ways:

Make pieces of foiled cardboard the same size as the oven sides and place these in the wall spaces.

Make a new reflector the size of the entire lid (see photo above).

Make the drip pan using sheet metal, such as aluminum flashing. Paint this black and elevate this off the bottom of the oven slightly with small cardboard strips.